The molecular cloning of subunit 1 of the platelet-derived growth factor (PDGF) is being attempted. The strategies used so far have been: (a) screening of humam genomic libraries with digonucleotides made on the presumptive nucleotide sequence reconstructed from the partial amio acid sequence availabel for PDGF-1; (b) immunological detection of PDGF-1 and identification of the PDGF 1 transcript to identify a candidate cell line for construction of a cDNA library. Normal mast cells, which display an absolute dependence for growth on interleukin (IL)-3, have been infected with several retroviruses in vitro. Infection with Harvey-, BALB- and Moloney-MSV is unable to relieve IL-3 dependence, whereas infection and transformation with Abelson murine leukemia virus (Av-MuLV) induces IL-3 independent cell growth.. The abrogation of IL-3 dependence is not due to an autocrine mechanism.